As already mentioned, paladins have been able to troll the tank with that for ages.

Personally I like things like this to spice up the heroics I run with friends for valour. Back in Wotlk we'd try and make them a challenge to spice things up by doing things like BoPing the tank, healer won't dispel anything / not use things like watershield etc.

You know, for a kitten that shits rainbows, you seem to like to stay inside the box.

Mine too, but it's harder to check that in the middle of combat, since there's usually a ton of buffs and they move around a lot.

(Also I generally assume that the average player, particularly those in LFR, would be using the default UI.)

So true. Most people in raid settings would be too busy doing other things than watching the numbers/text popping up above their heads showing what happened to them. They would be focusing more on fire, timers (boss abilities + their own cds/dots), positioning etc. I know I hardly ever look at my toon so animations on my toon is largely ignored by me.

Also we're talking about a PVE setting, not PVP where a lot more people will notice every scrolling text + animations on toons. More important in PVP than PVE imo since you need to watch what your opponent is doing and what colour/stance/shit glowing above his head in order to counter them.